The present invention relates to a method for associating a plurality of images when 3-dimensional measurement is performed by using a stereo image in photogrammetry (photographic surveying). The present invention also relates to a 3-dimensional data preparation system for preparing a 3-dimensional data from a plurality of images.
A stereo image is an image, which represents an image of an object in stereoscopic manner. The stereo image comprises a plurality of images taken on a same object from different image-taking directions. In the plurality of images, corresponding points on each of the images must be associated with each other. By associating in such manner, it is possible to obtain 3-dimensional data of an arbitrary point in the image.
In order to have a stereo image, from which 3-dimensional data can be obtained, matching (stereo-matching) between the images must be carried out. Therefore, for the images to be used in the photogrammetry, it is necessary to have an image, in which stereo-matching can be performed, and there must be a condition between the images, in which stereo-matching can be performed.
In order to perform the stereo-matching, parameters must be determined for each of images such as a position of a camera to take images, image magnification factor of the image, tilting (a horizontal angle ω of an optical axis of the camera, a vertical angle φ of the optical axis of the camera, and a twisting angle r around the optical axis of the camera). When these parameters are processed only based on the information from the images (digital image), 6 or more points are required, which indicate the same point in a plurality of images (this is called a control point or a pass point).
When a stereo image is prepared, by adding to each images the data of position of the camera and the data of azimuth when each image is taken, it is possible to convert an arbitrary point in the image from a coordinate system of the image to a coordinate system on the ground, i.e. 3-dimensional measurement can be performed.
A plurality of images, which constitute a stereo image, are the images, which are taken on the same object from different image-taking directions. In order to obtain 3-dimensional data, it is desirable that image-taking conditions are kept in constant state even when the image-taking direction is changed to facilitate stereo-matching. However, it is difficult to take image by maintaining height of a camera or tilting of an optical axis of the camera in constant state when the images are taken manually, and it requires a large-scale system.
When images are taken manually from a first point O1 to a second point O2 with a distance D between these points, magnification factor of the images and the image-taking posture are normally different in each case. As shown in FIG. 8, images 2 and 3 taken of an object 1 are different from each other in image magnification factor, tilting (a horizontal angle ω of an optical axis of a camera, a vertical angle φ of the optical axis of the camera, and a twisting angle r with the optical axis of the camera as the center). In such case, in order to perform stereo-matching between the images 2 and 3, pass points must be manually selected with high accuracy on each of the images. Further, because image-taking conditions are different from each other, selection and matching of the pass point P of the images 2 and 3 requires long time and high skill.